1. Field of the Invention
The present invention relates to a method of making a spin valve sensor with a controlled ferromagnetic coupling field (HF) wherein a pinned layer structure is sputter deposited in such a manner that a desired ferromagnetic coupling field (HF) can be obtained within a wider range of deposition times of a copper spacer layer.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer structure and a ferromagnetic free layer structure. An antiferromagnetic pinning layer interfaces the pinned layer structure for pinning a magnetic moment of the pinned layer structure 90° to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the magnetic disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer structure is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or bias point position in response to positive and negative magnetic field signals from a rotating magnetic disk. The quiescent position, which is parallel to the ABS, is the position of the magnetic moment of the free layer structure with the sense current conducted through the sensor in the absence of signal fields.
The thickness of the spacer layer is chosen so that shunting of the sense current and a ferromagnetic coupling field (HF) between the free and pinned layer structures is minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons are scattered at the interfaces of the spacer layer with the pinned and free layer structures. When the magnetic moments of the pinned and free layer structures are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. Changes in scattering changes the resistance of the spin valve sensor as a function of cos θ, where θ is the angle between the magnetic moments of the pinned and free layer structures. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in the resistance of the sensor as the magnetic moment of the free layer structure rotates from a position parallel with respect to the magnetic moment of the pinned layer structure to an antiparallel position with respect thereto and R is the resistance of the sensor when the magnetic moments are parallel.
In addition to the spin valve sensor the read head includes nonconductive nonmagnetic first and second read gap layers and ferromagnetic first and second shield layers. The spin valve sensor is located between the first and second read gap layers and the first and second read gap layers are located between the first and second shield layers. In the construction of the read head the first shield layer is formed first followed by formation of the first read gap layer, the spin valve sensor, the second read gap layer and the second shield layer. Spin valve sensors are classified as a top or a bottom spin valve sensor depending upon whether the pinning layer is located near the bottom of the sensor close to the first read gap layer or near the top of the sensor close to the second read gap layer. Spin valve sensors are further classified as simple pinned or antiparallel pinned depending upon whether the pinned layer structure is one or more ferromagnetic layers with a unidirectional magnetic moment or a pair of ferromagnetic layers that are separated by a coupling layer with magnetic moments of the ferromagnetic layers being antiparallel. Spin valve sensors are still further classified as single or dual wherein a single spin valve sensor employs only one pinned layer and a dual spin valve sensor employs two pinned layers with the free layer structure located therebetween.
The transfer curve of a spin valve sensor is defined by the aforementioned cos θ where θ is the angle between the directions of the magnetic moments of the free and pinned layers. In a spin valve sensor subjected to positive and negative magnetic signal fields from a moving magnetic disk, which are typically chosen to be equal in magnitude, it is desirable that positive and negative changes in the resistance of the spin valve read head above and below a bias point on the transfer curve of the sensor be equal so that the positive and negative readback signals are equal. When the direction of the magnetic moment of the free layer is substantially parallel to the ABS and the direction of the magnetic moment of the pinned layer is perpendicular to the ABS in a quiescent state (no signal from the magnetic disk) the positive and negative readback signals should be equal when sensing positive and negative fields that are equal from the magnetic disk. Accordingly, the bias point should be located midway between the top and bottom of the transfer curve. When the bias point is located below the midway point the spin valve sensor is negatively biased and has positive asymmetry and when the bias point is above the midway point the spin valve sensor is positively biased and has negative asymmetry. When the readback signals are asymmetrical, signal output and dynamic range of the sensor are reduced. Readback asymmetry is defined as
            V      1        -          V      2            max    ⁡          (                        V          1                ⁢                                                  ⁢                                                ⁢        or        ⁢                                  ⁢                  V          2                    )      For example, +10% readback asymmetry means that the positive readback signal V1 is 10% greater than it should be to obtain readback symmetry. 10% readback asymmetry is acceptable in some applications. +10% readback asymmetry may not be acceptable in applications where the applied field magnetizes the free layer close to saturation. The designer strives to improve asymmetry of the readback signals as much as practical with the goal being symmetry.
The location of the transfer curve relative to the bias point is influenced by four major forces on the free layer of a spin valve sensor, namely a ferromagnetic coupling field (HF) between the pinned layer and the free layer, a net demagnetizing (demag) field (HD) from the pinned layer, a sense current field (HI) from all conductive layers of the spin valve except the free layer and a net image current field (HIM) from the first and second shield layers if the sensor is offset between the first and second read gap layers. In order to reduce demagnetizing field from the pinned layer on the free layer, the pinned layer may be an antiparallel (AP) pinned layer structure. An AP pinned layer structure has an antiparallel coupling (APC) layer which is located between ferromagnetic first and second AP pinned layers. The first and second AP pinned layers have magnetic moments which are antiparallel with respect to one another because of the strong antiferromagnetic coupling therebetween. The AP pinned layer structure is fully described in commonly assigned U.S. Pat. No. 5,465,185 which is incorporated by reference herein. Because of the partial flux closure between the first and second AP pinned layers of each first and second AP pinned structures, each AP pinned layer exerts only a small demagnetizing field on the free layer. Because of the small demagnetizing field the exchange coupling between the AP pinned layer structure and the pinning layer is increased for promoting high stability when the spin valve sensor is subjected to unwanted magnetic fields in the presence of elevated temperatures.
The aforementioned dual spin valve sensor includes a ferromagnetic free layer structure between nonmagnetic electrically nonconductive first and second spacer layers which are, in turn, located between ferromagnetic first and second pinned layer structures. The spacer layers are typically copper (Cu) and the pinned layers are typically cobalt iron (CoFe). It has been found that a cobalt iron pinned layer next to a copper spacer layer promotes the magnetoresistive coefficient dr/R of the sensor. The dual spin valve sensor is desirable because its magnetoresistive coefficient dr/R is about 1.5 times greater than the magnetoresistive coefficient dr/R of a single spin valve sensor.
The dual spin valve sensor is also desirable from the standpoint that there is about an equal amount of conductive material on each side of the free layer structure so that the sense current field (HI) acting on the free layer structure is essentially zero. It is also desirable that each of the pinned layer structures in the dual spin valve sensor be an antiparallel (AP) pinned layer structure, as discussed hereinabove. Each of the AP pinned layer structures has a low demagnetizing field and the thicknesses of the AP pinned layers of the AP pinned layer structures can be designed so that these demagnetizing fields have nearly total flux closure. Accordingly, the net demagnetizing field from the first and second AP pinned layer structures can be essentially zero. In a preferred embodiment the dual spin valve sensor is not offset between the first and second read gap layers so that adequate insulation can be provided for preventing shorts between the lead layers of the sensor and the first and second shield layers while maintaining a minimum read gap between the first and second shield layers for promoting linear read bit density. Accordingly, virtually the only magnetic field urging the magnetic moment of the free layer structure from a position parallel to the ABS is a net ferromagnetic coupling field (HF) between each of the first and second AP pinned layer structures and the free layer structure.
In present methods of sputter depositing the copper spacer layers the ferromagnetic coupling field cannot be controlled within a desirable range from 0 to −10 Oe. As an example, when the deposition time for the first copper spacer layer is 26 seconds the ferromagnetic coupling field (HF) is −20 Oe and when the deposition time for the first copper spacer layer is 25 seconds the ferromagnetic coupling field (HF) is +10 Oe. Within a one second deposition time the ferromagnetic coupling field (HF) has a difference of 30 Oe. Because of process variations, it is very difficult to obtain the aforementioned desirable range of 0 Oe to −10 Oe when the range of the ferromagnetic coupling field (HF) is 30 Oe for only one second of copper deposition time. The same problem occurs for a single AP pinned layer structure or a single spin valve sensor with a single pinned layer structure. However, in these embodiments a net sense current field and a net demagnetizing field are present for help in properly biasing the free layer structure.
Another problem that arises, especially with the use of tunnel junction sensor (tunnel valves) is that undesirably high coercivity in the free layer reduces the sensitivity and performance of the sensor. A tunnel valve operates based on the spin dependent tunneling of electrons through a non-magnetic, electrically insulating barrier layer. The tunnel valve is constructed with free and pinned layers similar to GMR senor, however, rather than having an electrically conductive spacer layer, an electrically insulating barrier layer is sandwiched between the free and pinned layers. Unfortunately, the high free layer coercivity in a tunnel valve results in less than desirable tunnel valve sensitivity and performance.
Therefore, there is a strong felt need for a tunnel valve having reduced magnetic coercivity in its free layer. Such reduced coercivity would greatly improve the performance of such a tunnel valve sensor.